This invention relates to vehicle mounted gas detectors using laser absorption cells.
Vehicle mounted gas sensors pose difficult problems not found with stationary sensors. In particular, for aircraft mounted gas sensors the difficult environmental conditions make accurate and reliable measurement difficult. Aircraft and other fast moving vehicles are subject to mechanical disturbance such as from rapid temperature variations, from xe2x88x9250xc2x0 C. to 30xc2x0 C., high wind speed, high vibrations and impact from fast moving objects, including birds, flies, ice pellets, rain and snow. Aircraft, other fast moving vehicles and in particular helicopters are also subject to electrical interference. The design of a robust and sensitive mobile gas sensor poses a difficult design challenge.
Canadian patent application no. 2,219,335 published Nov. 24, 1997 shows a prior art laser absorption cell for a gas detector. The gas detector is designed for use with aircraft and has a laser absorption cell of the Herriot type carried on the exterior of the aircraft. This laser absorption cell was used for example to detect methane leaks from pipelines. A laser transceiver and analyser module mounted on board the aircraft is coupled to the laser absorption cell through multi-mode fiber optics. Use of multi-mode fiber optics caused optical noise and reduced detection sensitivity. To reduce optical noise, detection of methane took place in the 1300 nm absorption wavelength since that wavelength was less affected by the telecommunication fiber optic. However, since absorption of light in the methane absorption band at 1300 nm is relatively weak, multiple passes of the Herriot cell were required to obtain a suitable strong signal. Due to the multiple passes of light across the Herriot cell, and the necessity of collimating the light into an optical fiber for delivery of light to the laser receiver, the gas detector was sensitive to thermal and mechanical misalignment that decreased the detector""s reliability and required high maintenance. Reduced reliability and high maintenance of the gas detector restricted its use to the rental market.
To overcome problems of the prior art gas detector, a gas detector is now proposed according to an aspect of the invention that uses a photo-detector mounted with the laser absorption cell exterior to the vehicle to detect light that has made at least one pass of the laser absorption cell.
Therefore, there is provided, according to an aspect of the invention, a laser transmitter and signal analyser carried on, and preferably within, a vehicle, the vehicle having an exterior, a laser absorption cell carried on the exterior of the vehicle, a light guide connecting light from the laser transmitter into the laser absorption cell, a photo-detector mounted with the laser absorption cell exterior to the vehicle to convert light that has traversed the laser absorption cell into electrical signals, and a cable connecting the photo-detector to the signal analyser.
According to a further aspect of the invention, there is provided a method of detecting a target gas, the method comprising the steps of:
traversing a target area with a vehicle having an exterior;
passing air through a laser absorption cell carried on the exterior of the vehicle;
directing light through the laser absorption cell;
converting light that has made at least one pass through the laser absorption cell into electrical signals using a photo-detector mounted with the laser absorption cell exterior to the vehicle; and
analysing the electrical signals for a signal indicative of the presence of the target gas.
According to a further aspect of the invention, a laser absorption cell, in which light emitted from a light guide traverses the cell and is collected by a photo-detector, is provided with a retro-reflector to reflect light that has entered the light absorption cell from the light guide towards the photo-detector. The retro-reflector accommodates misalignment of the light guide and photo-detector in the high vibration environment of a fast moving vehicle.
According to a further aspect of the invention, a mirror is mounted on the frame to reflect light from the light guide across the light absorption cell in at least two back and forth passes before the light reaches the photo-detector.
According to a further aspect of the invention, the gas detector further comprises a protective window mounted on the frame over the retro-reflector to protect the retro-reflector from airborne contaminants.
In one optical arrangement, light from the light guide traverses the laser absorption cell reflected off a mirror across the cell, and is directed onto the photo-detector, for example by a collecting mirror such as an offset parabolic mirror.
In another optical arrangement, light from the light guide traverses the laser absorption cell to a retro-reflector and reflected from the retro-reflector onto a photo-detector.
The mobile gas detector is primarily used for airborne detection. Use of a photo-detector allows detection of methane using light within the methane absorption band at 1650 nm. The photo-detector is preferably a photo-diode operating in photo-voltaic mode to avoid the need for an external power supply for the photo-detector.
These and other aspects of the invention are described in the detailed description of the invention and claimed in the claims that follow.